It is well known in the laser field that lasers may employ gain media with one or other forms of large area geometry. In one known example of a large area gain medium, the axial dimension thereof is some multiple of the width, which in turn is substantially larger than the thickness, so forming a slab of rectangular cross-section. U.S. Pat. No. 4,719,639 by Tulip and U.S. Pat. No. 4,930,138 by Opower, both describe carbon dioxide lasers employing slab-shaped gain media. In both cases large planar metal or dielectric surfaces are provided at the boundaries of the slab-shaped gain media and the dimensions of the slab-shaped gain media are such as to allow optical waveguiding between these surfaces. For convenience such an arrangement is referred to herein and by those skilled in the art as a slab waveguide.
It is well known that the successful use of high power lasers for applications such as material processing and laser radar depends on achieving high optical quality in the beam generated by the laser. In a slab waveguide laser, the physical separation of the major planar surfaces defining the optical waveguide is usually selected so that the component of the optical field in the direction at right angles to these surfaces (the transverse direction) propagates in a low-order mode of the waveguide. Thus, for example, surface separations in the approximate range of 0.5 mm-3.0 mm are suitable for low-order waveguide propagation of radiation at the 10 microns wavelength produced by carbon dioxide lasers.
It is also essential to achieve adequate control over the characteristics of the intra-cavity optical field in the plane parallel to the major planar surfaces of the optical waveguide (the lateral direction), if a laser output beam exhibiting the required propagation and focusing characteristics is to be obtained. The lateral modes of a slab waveguide laser are well described by high order rectangular waveguide modes, but in general an incoherent mode mixture is observed with multiple output frequencies. A single high order mode can only be observed at much reduced output power, by substantially increasing the cavity losses.
It is well-known that if a coherent optical field is incident upon a periodic transverse (grating-like) structure, the optical field distribution at the structure will be exactly reproduced at other planes transverse to the axis of propagation located at distances that are integer multiples of the distance Z.sub.T given by: ##EQU1## where Z.sub.T is referred to as the Talbot distance, d is the spacing of the individual apertures in the periodic transverse structure, .lambda. is the wavelength, and n is the refractive index of the medium. This is referred to as the Talbot effect. The Talbot effect produces successive images without the aid of lenses.
This so-called coherent imaging property of periodic structures has been used in the prior art as the basis of a technique for producing phase-locked or coherent operation of an array of multiple individual adjacent parallel laser oscillators, spatially arranged in a one-dimensional or two-dimensional geometric configuration. In such cases, phase-locking of the array elements has been achieved by arranging that light from a given element of an array of laser gain media couple into adjacent elements by diffraction within a common extended laser cavity. By selecting the round trip distance from the output plane of the array elements to the common laser mirror, which constitutes the diffractive coupling zone, to be a Talbot half distance or multiple thereof, the periodic field in the output plane of the array is re-imaged back upon itself after the round trip to the laser mirror. Thus, it is possible to increase the effectiveness of diffractive coupling between the array elements because of the self-imaging properties associated with the Talbot effect.